1. Field of the Invention
The invention relates to a process for fabricating a light-weight gelatin-based hologram layer on substrates, and, in particular, to a process for increasing the abrasive and atmospheric moisture resistance of the gelatin without the need for a cover plate.
2. Description of the Prior Art
It has been widely recognized in the photographic art that a light-sensitive photographic emulsion, or photosensitive material, applied directly to the surface of a plastic support or substrate does not exhibit sufficient adhesion to the support film for use in most of the usual photographic end uses. As a result, it has become common practice to interpose one or more "subbing" layers between the support film and the photographic emulsion or photosensitive material. These problems also obtain for holographic applications, since photosensitive materials applied to plastic substrates are also employed.
There are many problems attending the use of chemical subbing layers on photographic and holographic plastic substrates. In addition to the obvious drawbacks of having to deposit one or more subbing layers, these subbing layers:
(1) are specific to plastics of different chemical composition;
(2) do not prevent the diffusion of water vapor into the photosensitive layer and therefore do not improve hologram stability; and
(3) can be grainy and non-uniform in thickness, which graininess causes light scattering noise during exposure and which non-uniformity causes phase errors in transmitted light.
Chemical subbing techniques for photographic plastic substrates are the subject of a review in a book by G. F. Duffin, Photographic Emulsion Chemistry, Focal Press Limited, London, 1966. A technique for chemically subbing poly-(methyl methacrylate) for use as a dichromated gelatin holographic substrate is the subject of an article by D. G. McCauley et al., in Vol. 12, Applied Optics, pp. 232-242 (1973).
While the subbing layers of the prior art are generally suitable for regular photographic substrates, the graininess and non-uniformity limits their usefulness for fabricating holographic substrates.
An additional problem involving holographic substrates has been the need for a cover plate to protect the photosensitive layer. In certain requirements, such as helmet-mounted displays, the weight of the cover plate, even though of light-weight plastic, is still sufficient to cause strain on the wearer. Further, the thickness of the cover plate is often sufficient to create ghost images.
Subbing layers are not required when using glass plates as substrates, as is well-known, because the photosensitive layer usually adheres well to the glass. Indeed, it is largely due to the requirements of minimal weight that plastic substrates are employed. Nevertheless, there are instances where the use of glass substrates is dictated, yet, again because of weight or ghost images, glass cover plates are not desirable.